A method that improves the luminous efficiency of an organic EL device by adjusting the optical interference distance has been widely used. A method that adjusts the emission spectrum of a top-emission organic EL device by providing organic layers including an emitting layer between a metal electrode and a semitransparent metal electrode to form an optical resonator has been used as a means that improves the luminous efficiency, provides a sharp emission spectrum, and adjusts the peak wavelength. The length of the resonator is adjusted by changing the distance between the metal electrode and the semitransparent metal electrode. However, a change in carrier balance occurs when adjusting the optical interference by changing the thickness of the organic layer, so that a change in luminous efficiency occurs. Therefore, since the thickness of the organic layer is restricted by the conditions that maintain the carrier balance, the length of the resonator cannot be adjusted arbitrarily.
The optical interference distance of the top-emission organic EL device may be adjusted without causing a change in carrier balance by providing a thin film structure that allows an adjustment of the optical interference distance on the upper semitransparent metal electrode. In Patent Document 1, a resonator is formed on the upper electrode, and the peak wavelength is adjusted by adjusting the length of the resonator so that the optical length of the device is equal to an integral multiple of one half-wavelength (e.g., Fabry-Perot resonator). Patent Document 2 discloses a method that adjusts the optical interference distance by forming an optical adjustment layer having a thickness of several to several hundred nanometers on the upper electrode.
Accordingly, it is known in the art to adjust the optical interference distance of the top-emission organic EL device by providing a thin film structure having a thickness of several to several hundred nanometers or several micrometers on the upper electrode so that the optical length is equal to an integral multiple of one half-wavelength (e.g., Fabry-Perot resonator).
It has been proposed to improve the luminous efficiency of the top-emission organic EL device by providing a thin film structure referred to as a resonator, a protective film, a capping layer, or an optical adjustment layer on the upper semitransparent electrode. However, a method that achieves a high luminous efficiency, a sharp emission spectrum, and a short peak wavelength in the blue emission region (wavelength: 400 to 500 nm) has not been proposed. In particular, since blue light has a short wavelength and high energy as compared with green light and red light, it has been difficult to optimize the optical interference distance while maintaining the carrier balance.
The emission properties may be improved as follows by providing a thin film structure on the upper semitransparent electrode, for example. In Patent Document 2, an organic capping layer having a refractive index of 1.7 or more and a thickness of 600 angstroms is formed on the upper electrode of the top-emission die organic EL device so that emission from the red-emitting device and the green-emitting device is improved by a factor of about 1.5.
In Patent Document 3, an organic capping layer doped with Nile red is formed on the upper electrode of the top-emission organic EL device in order to suppress reflection of external light, and achieve high contrast due to an increase in luminous efficiency.
Patent Document 4 discloses a capping layer having an energy gap of 3.2 eV or more, a refractive index n of higher than 1.75, and an extinction coefficient k of less than 0.12. The capping layer having an energy gap of 3.2 eV (corresponding to a wavelength of 387 nm) or more exhibits excellent transparency over almost the entire visible region (wavelength: 380 to 780 nm). Therefore, the luminous efficiency increases due to the capping layer over the entire visible region.
Patent Document 4 states that a material having an energy gap of less than 3.2 eV is not desirable since such a material affects the blue wavelength.